Reproduction may be divided into two main classes, namely, sexual and asexual. The latter is effected in many ways—by gemmation, that is by the formation of buds of various kinds, and by fissiparous generation, that is by spontaneous or artificial division. It is notorious that some of the lower animals, when cut into many pieces, reproduce so many perfect individuals: Lyonnet cut a Nais or freshwater worm into nearly forty pieces, and these all reproduced perfect animals.[[874]] It is probable that segmentation could be carried much further in some of the protozoa, and with some of the lowest plants each cell will reproduce the parent-form. Johannes Müller thought that there was an important distinction between gemmation and fission; for in the latter case the divided portion, however small, is more perfectly organised; but most physiologists are now convinced that the two processes are essentially alike.[[875]] Prof. Huxley remarks, "fission is little more than a peculiar
mode of budding," and Prof. H. J. Clark, who has especially attended to this subject, shows in detail that there is sometimes "a compromise between self-division and budding." When a limb is amputated, or when the whole body is bisected, the cut extremities are said to bud forth; and as the papilla, which is first formed, consists of undeveloped cellular tissue like that forming an ordinary bud, the expression is apparently correct. We see the connection of the two processes in another way; for Trembley observed that with the hydra the reproduction of the head after amputation was checked as soon as the animal began to bud.[[876]]
Between the production, by fissiparous generation, of two or more complete individuals, and the repair of even a very slight injury, we have, as remarked in a former chapter, so perfect and insensible a gradation, that it is impossible to doubt that they are connected processes. Between the power which repairs a trifling injury in any part, and the power which previously "was occupied in its maintenance by the continued mutation of its particles," there cannot be any great difference; and we may follow Mr. Paget in believing them to be the selfsame power. As at each stage of growth an amputated part is replaced by one in the same state of development, we must likewise follow Mr. Paget in admitting "that the powers of development from the embryo are identical with those exercised for the restoration from injuries: in other words, that the powers are the same by which perfection is first achieved, and by which, when lost, it is recovered."[[877]] Finally, we may conclude that the several forms of gemmation, and of fissiparous generation, the repair of injuries, the maintenance of each part in its proper state, and the growth or progressive development of the whole structure of the embryo, are all essentially the results of one and the same great power.
Sexual Generation.—The union of the two sexual elements seems to make a broad distinction between sexual and asexual reproduction. But the well-ascertained cases of Parthenogenesis prove that the distinction is not really so great as it at first appears; for ovules occasionally, and even in some cases
frequently, become developed into perfect beings, without the concourse of the male element. J. Müller and others admit that ovules and buds have the same essential nature. Certain bodies, which during their early development cannot be distinguished by any external character from true ovules, nevertheless must be classed as buds, for though formed within the ovarium they are incapable of fertilisation. This is the case with the germ-balls of the Cecidomyide larvæ, as described by Leuckart.[[878]] Ovules and the male element, before they become united, have, like buds, an independent existence.[[879]] Both have the power of transmitting every single character possessed by the parent-form. We see this clearly when hybrids are paired inter se, for the characters of either grandparent often reappear, either perfectly or by segments, in the progeny. It is an error to suppose that the male transmits certain characters and the female other characters; though no doubt, from unknown causes, one sex sometimes has a stronger power of transmission than the other.
It has been maintained by some authors that a bud differs essentially from a fertilised germ, by always reproducing the perfect character of the parent-stock; whilst fertilised germs become developed into beings which differ, in a greater or less degree, from each other and from their parents. But there is no such broad distinction as this. In the eleventh chapter, numerous cases were given showing that buds occasionally grow into plants having new and strongly marked characters; and varieties thus produced can be propagated for a length of time by buds, and occasionally by seed. Nevertheless, it must be admitted that beings produced sexually are much more liable to vary than those produced asexually; and of this fact a partial explanation will hereafter be attempted. The variability in both cases is determined by the same general causes, and is governed by the same laws. Hence new varieties arising from buds cannot be distinguished from those arising from seed. Although bud-varieties usually retain their character during
successive bud-generations, yet they occasionally revert, even after a long series of bud-generations, to their former character. This tendency to reversion in buds is one of the most remarkable of the several points of agreement between the offspring from bud and seminal reproduction.
There is, however, one difference between beings produced sexually and asexually, which is very general. The former usually pass in the course of their development from a lower to a higher grade, as we see in the metamorphoses of insects and in the concealed metamorphoses of the vertebrata; but this passage from a lower to a higher grade cannot be considered as a necessary accompaniment of sexual reproduction, for hardly anything of the kind occurs in the development of Aphis amongst insects, or with certain crustaceans, cephalopods, or with any of the higher vascular plants. Animals propagated asexually by buds or fission are on the other hand never known to undergo a retrogressive metamorphosis; that is, they do not first sink to a lower, before passing on to their higher and final stage of development. But during the act of asexual production or subsequently to it, they often advance in organisation, as we see in the many cases of "alternate generation." In thus speaking of alternate generation, I follow those naturalists who look at the process as essentially one of internal budding or of fissiparous generation. Some of the lower plants, however, such as mosses and certain algæ, according to Dr. L. Radlkofer,[[880]] when propagated asexually, do undergo a retrogressive metamorphosis. We can to a certain extent understand, as far as the final cause is concerned, why beings propagated by buds should so rarely retrogress during development; for with each organism the structure acquired at each stage of development must be adapted to its peculiar habits. Now, with beings produced by gemmation,—and this, differently from sexual reproduction, may occur at any period of growth,—if there were places for the support of many individuals at some one stage of development, the simplest plan would be that they should be multiplied by gemmation at that stage, and not that they should first retrograde in their development to an earlier or simpler structure, which might not be fitted for the surrounding conditions.
From the several foregoing considerations we may conclude that the difference between sexual and asexual generation is not nearly so great as it at first appears; and we have already seen that there is the closest agreement between gemmation, fissiparous generation, the repair of injuries, and ordinary growth or development. The capacity of fertilisation by the male element seems to be the chief distinction between an ovule and a bud; and this capacity is not invariably brought into action, as in the cases of parthenogenetic reproduction. We are here naturally led to inquire what the final cause can be of the necessity in ordinary generation for the concourse of the two sexual elements.